Hydrocarbons, such as oil and gas, are commonly obtained from subterranean formations that may be located onshore or offshore. The development of subterranean formations and the processes involved in removing hydrocarbons from a subterranean formation typically involve a number of different steps such as, for example, drilling a wellbore at a desired well site, treating the wellbore to optimize production of hydrocarbons, and performing the necessary steps to produce and process the hydrocarbons from the subterranean formation.
Upon drilling a wellbore that intersects a subterranean hydrocarbon-bearing formation, a variety of downhole tools may be positioned in the wellbore during exploration, completion, production, and/or remedial activities. Some downhole tools are outfitted with sensors for detecting various properties of the wellbore. One available type of downhole tool is a nuclear magnetic resonance (NMR) logging tool, which can be used to measure certain properties of the geological formation through which the wellbore is drilled.
NMR tools generally include a large permanent magnet used to impose a static magnetic field downhole to preferentially align certain nuclei in the formation and thereby produce a bulk magnetization. After a change in the static field, the nuclei converge upon their equilibrium alignment with a characteristic exponential relaxation time constant known as the “spin-lattice” or “longitudinal” relaxation time T1. Another relaxation time constant that can be measured is the “spin-spin” or “transverse” relaxation time T2. The tool applies a radio frequency electromagnetic pulse whose magnetic component is perpendicular to the static field produced by the permanent magnet. This pulse tips the nuclei's magnetic orientation into the transverse (perpendicular) plane and, once the pulse ends, causes them to precess (“spin”) in the transverse plane as they realign themselves with the static field. This T2 relaxation time constant represents how quickly the transverse plane magnetization disperses through de-phasing and magnitude loss. The precessing nuclei generate a detectable radio frequency signal that can be used to measure statistical distributions of T1 and T2, from which other formation properties such as porosity, permeability, and hydrocarbon saturation can be determined.
When such NMR logging tools are deployed into a wellbore, they generally must first pass through a section of casing. This casing is often made of magnetic steel. During this process, an attractive force may be generated between the steel casing and the large permanent magnet in the NMR tool. Such attractive force between the casing and the NMR tool can keep the NMR tool from dropping down the wellbore, especially in deviated wells. NMR tools used in large wellbores are equipped with a pad to reduce the magnetic attractive force between the tool and the casing. Unfortunately, this type of pad can add undesirable bulk to the NMR tool, making the tool unsuitable for use in smaller wellbores.